The compression scheme of text image was conventionally implemented using the MMR (Modified Modified Read) or JBIG (Joint Bi-level Image Coding Experts Groups) features that process the text image as binary image. The reason why a text image is handled as a binary image may be the nature of the text image that allows image processing by identifying the background of the text image from the text region.
The standards of the MMR or JBIG are developed focusing on application to facsimile (referenced document: ISO/IEC JTC1/SC29/WG1 N1359, “14492 FCD Information Technology—Coded Representation of Picture and Audio Information—Lossy/Lossless Coding of Bi-level Images”, JBIG Committee (Jul. 16, 1999)). At the current stage, they are substantially the standard scheme for binary image compression. In the compression schemes of MMR, JBIG and the like, reusability is contemplated through modularization by hardware or software. The hardware device particularly has high usage efficiency in apparatuses with poor CPU (Central Processing Unit) power such as portable information terminals that have difficulty in effecting image compression processing by software from the standpoint of the processing rate.
In contrast, compression of a text image formed of a gray scale image was carried out by the scheme of dividing a gray scale image into a plurality of weighted bit planes, each represented as a binary image, to divide the gray scale image into a plurality of binary images for processing. Such an example is shown in FIG. 1. FIG. 1 corresponds to an octal image in which each pixel can take any eight values (0–7). FIGS. 2–4 correspond to the division of the octal image into three planes. Each plane corresponds to each digit of the original image represented in binary of three digits. Here, the value of each plane is determined depending upon whether each digit is 0 or 1. The first plane shown in FIG. 2 corresponds to the most significant digit of the three digits. The second plane shown in FIG. 3 corresponds to the second most significant digit. The third plane shown in FIG. 4 corresponds to the least significant digit.
By dividing a gray scale image into binary bit planes, the binary image compression scheme can be employed in the compression of a gray scale image.
According to the aforementioned conventional compression scheme, each of the first to third planes is compressed into a binary image independent of other planes. There was a problem that the amount of compression increases since the correlation between respective planes is not utilized. For example, in the case where an image constituted by just the value of 0 (represented as 000 in binary of 3 digits) and the value of 7 (represented as 111 in binary of 3 digits) is to be compressed from an octal image that can take any of the 8 values of 0–7, the contents of the first to third planes are all identical according to the aforementioned bit plane generation method. Even though there is only information of one plane, the compression size increases to as much as three times thereof since each plane is compressed independently.
Although this is an extreme example, there is a definite correlation between each plane also in a general text image. Although the first plane shown in FIG. 2 is slightly degraded in picture quality, the contents of the original text can be generally read. The second plane shown in FIG. 3 and the third plane shown in FIG. 4, though gradually degraded in picture quality, can be recognized to a level so as to work out the original text or so as to locate the characters in the original text. This means that the pixel values of respective pixels corresponding to each plane are not independent, i.e., have a correlation.
A compression scheme utilizing the correlation between bit planes to solve the above problem is disclosed in Japanese Patent Laying-Open No. 63-296564. According to this compression scheme, the most significant plane is coded by a first coding method and the planes of the lower level are coded with linear prediction using pixels of bit planes already coded or pixels already coded in the relevant bit plane.
Although the problem of the compression size being increased can be alleviated, this compression scheme does not correspond to the binary image compression standard scheme of MMR, JBIG, or the like. This imposes the problem that the currently-available hardware or software modules cannot be reused.